Yep, that has been observed many times. In contrast to rendering applications, physics simulations will not use all cores. Also to be considered, splitting tasks in many threads may actually reduce the overall execution of the simulation because it creates overhead. The best hardware for realflow is a few cores, but high clock speed.

I agree. But the trend seems rather to move the entire simulation to the GPU (dyverso solver) instead of better use of more CPU cores. The hardware of a GPU is better optimized for parallel processing than the CPU with relatively remote RAM.